Novel immunotherapies and cancer vaccines are being developed. The success of these therapies requires an immunocompetent host. Immune suppression occurs in many cancer patients and is a major impediment for developing successful cancer immunotherapies. Although there are numerous types of immune suppression, tumor-induced Myeloid Suppressor Cells (MSC), also known as Immature Myeloid Cells, are found in many patients and in animals with transplanted and spontaneous tumors. We have recently identified in mice a gene, the Signal Transducer and Activator of Transcription 6 (STAT6) gene, that when deleted, results in greatly improved survival and immune rejection of established metastatic mammary carcinoma following surgical removal of primary tumor. Effective tumor-immunity in post-surgery STAT6- deficient mice is mediated by three components: 1) The generation of M1 type macrophages; 2) The generation of tumor-specific CD8+ T cells; and 3) The rapid decrease to baseline in MSC levels. Because MSC accumulation and retention inhibit tumor-specific immunity and interfere with active immunotherapy, we will examine the mechanisms underlying STATG-induced retention of MSC in tumor-bearing mice. We propose the following three Specific Aims to accomplish this goal: 1) Myeloid suppressor cells (MSC) are potent inhibitors of CD4+ and CD8+ T lymphocytes that effectively block tumor-specific immunity. We will identify the ligand/receptor combination responsible for the post-surgery retention of 4T1-induced myeloid suppressor cells. 2) We have previously shown that IFNy is required for the rapid regression of MSC in post- surgery STAT6-deficient mice. We will determine the mechanism responsible for this regression, and will clarify the role of IFN^ in this process. 3) The pro-inflammatory cytokine IL-1/? causes excessive accumulation and retention of MSC in post-surgery mice. We will determine how IL-1[unreadable]regulates MSC levels, and ascertain if the link between inflammation and cancer is the induction of MSC. 4) MSC are found in many cancer patients and are thought to be an impediment to immune surveillance and immunotherapy. Using Gemcitabine, a drug that has recently been shown to down-regulate MSC, we will determine if reduction/elimination of MSC by itself is sufficient to mediate tumor rejection and if elimination of MSC impacts M1 macrophages and T lymphocytes. We have hypothesized that MSC block immunosurveillance, thereby facilitating the outgrowth of malignant cells. A better understanding of the regulation of tumor- induced immune suppression may reveal methods for controlling these cells in cancer patients, and thereby contribute to the development of effective cancer immunotherapies.